Some kind of craft? Friendship bracelet, bead kits, sketchbook...Books, Nintendo DS, word searches, what about those games where you jump the peg and try to wind up with only 1 peg left? How about a small bottle of bubbles? I know some other girls about her age that still love to blow bubbles! Make her a surprise bag so she does not know what you have, once she is done fishing, give it to her. If it is a craft, make sure it is instructions she understands, or something she has done before so you don't have to break away to help her with it if you do not want to. Let her take photos with the camera. Maybe get her an album and stickers to make an album for the family of your trip?
I agree, I'd be worried about leaving her. Are you on a big enough boat she wil have some shade, and room for projects? It is hard to see a Nintendo DS screen in the bright sun.
What about writing in a journal? Even if you have to make her slips of paper with a topic? I did this for my daughter at Christmas.
The iPod should last several hours, you can buy other chargers that do not go in the computer. Also, what about getting her a book for her iPod? Will she listen to a story?